We studied with the electron microscope distribution of lipoprotein lipase (LPL) in heart and brown adipose tissue of new born normal and lipoprotein lipase-deficient (cld/cld) mice. Bovine-LPL antiserum raised in chickens was used as the primary immunoreactant and rabbit anti-chicken IgG labelled with ferritin or gold particles as the secondary antibody. LPL was localized over capillary endothelium in normal heart and brown adipose tissue. Label was also found over endothelial projections and along the luminal surface of capillaries. Ferritin label was found in myocytes of normal heart in packets within intracellular channels near mitochondria and in small vesicles near the surface of cells. Very little label was found at the basal surface of endothelium or myocytes. Brown adipose tissue and heart of cld/cld mice contain 3-5 X normal amounts of inactive LPL-like protein. Ferritin label was found over larger peripheral vacuoles in both adipocytes and myocytes of cld/cld mice. This finding may reflect impaired transport of LPL to capillary endothelium. Electron microscopic studies showed that capillaries in heart, brown adipose tissue and heart of cld/cld mice were filled with large irregularly shaped chylomicrons, and parenchymal cells in these tissues contained subnormal amounts of triacylglycerol, relfecting absence of LPL activity in capillaries. Effect of pH on visualization of fatty acids as myelin figures was studied in adipose tissue undergoing hormone-stimulated lipolysis. Myelin figures were seen, using the freeze fracture technique, only in tissues incubated at pH 7.8-9.O. Lipoprotein lipase activity was demonstrated at the surface of cultured mouse macrophages (J774.1) by the presence of surface-attached remnants of triacylglycerol-rich particles in incubated-unfixed cells, and development of myelin figures at sites of surface-attached particles in incubated glutaraldehyde-fixed cells.